The purpose of this research is an attempt to understand how hyperthermia sensitizes radiation induced damage. Mutagen sensitive strains of Drosophila melangaster are subjected to hyperthermia treatment, 38 C for 1 hour, and then exposed to X-rays or gamma radiation. The strains are mutagen sensitive because of interference with some aspect of repair. What strain does not respond to hyperthermia effect upon radiation? The mutant may already have inhibited the repair similar to what hyperthermia would affect. The hyperthermia has been shown in our earlier investigation to increase radiation induced, dominant lethals, recessive sex-linked and the loss of the X and Y chromosome in those broods which represent cells in spermatogenesis in or about meiosis. We have obtained information that hyperthermia enhances gross delections of the X-chromosome. This and the above listed genetic aberrations will be investigated in 10 different mutagen sensitive stocks with respect to hyperthermia and radiation induced damage.